Field of the Invention
The present invention relates to a method for producing a foamed molded product with a pressurized fluid used as a physical foaming agent.
Description of the Related Art
In recent years, various types of injection molding methods and extrusion molding methods have been investigated, in which any pressurized fluid is used. Examples of the pressurized fluid include pressurized carbon dioxide and pressurized nitrogen. In the case of the molding method as described above, a fluid having an extremely high pressure is introduced into a molten resin, and hence it is possible to produce molded products having various functions. For example, in order to mutually compatibly dissolve polymers which are incompatible with each other, an injection molding method and an extrusion molding method are suggested for a polymer alloy, wherein molten resins and high pressure carbon dioxide are brought in contact and kneaded with each other in a plasticizing cylinder (Patent Literature 1: Japanese Patent Application Laid-open No. 2003-94477 and Non-Patent Literature 1: Proceedings of 17th JSPP Symposium of Japan Society of Polymer Processing, 227 (2009)). In the molding methods as described above, the molten resin and pressurized carbon dioxide are brought in contact and kneaded with each other by a kneading apparatus which is provided with a screw contained in a plasticizing cylinder.
On the other hand, a molding method is suggested, wherein supercritical carbon dioxide is introduced into a molten resin at an intermediate position of an extrusion machine having a vent portion in order to remove any hardly volatile component from a thermoplastic resin (Patent Literature 2: Japanese Patent Application Laid-open No. 11-292921). Further, an injection molding method is suggested, wherein a molten resin of thermoplastic resin is injected and charged into a mold from a plasticizing cylinder, and then a pressurized fluid, which contains supercritical carbon dioxide and a functional material such as an organic metal complex or the like, is introduced into the mold to thereby produce a thermoplastic resin molded product including the functional material dispersed on a surface (Patent Literature 3: Japanese Patent No. 3964447).
In the meantime, the solubility of pressurized carbon dioxide with respect to the resin is low. Therefore, in the case of the molding method including the step of bringing the molten resin and the pressurized carbon dioxide in contact with each other as described above, it is difficult to allow a large amount of pressurized carbon dioxide and the molten resin to be brought in contact and kneaded with each other. Therefore, in a case that the functional material is used together with pressurized carbon dioxide, it is also difficult to introduce the functional material into the molten resin at a high concentration. From such a viewpoint, a method for producing a molded product is suggested, wherein an kneading apparatus, in which an introducing port for introducing pressurized carbon dioxide is provided on an upper side surface of a plasticizing cylinder and a vent is provided on the downstream side from the introducing port, is used so that a molten resin, pressurized carbon dioxide, and a functional material are brought in contact and kneaded with each other in the plasticizing cylinder, and then the resin internal pressure of the molten resin is lowered before being injected and charged into a mold to separate only gasified carbon dioxide from the molten resin and discharge carbon dioxide from the vent (Patent Literature 4: Japanese Patent Application Laid-open No. 2009-298838 which corresponds to US2011/104380). According to this molding method, it is possible to improve the concentration of the functional material to be introduced into the molten resin, while controlling the concentration of pressurized carbon dioxide in the molten resin.
Further, in recent years, a foam injection molding method (producing method for producing a foamed molded product), using a physical foaming agent such as nitrogen and carbon dioxide in a supercritical state as the pressurized fluid, is researched and practically used (Patent Literature 5: Japanese Patent No. 2625576 which corresponds to WO92/17533; Patent Literature 6: Japanese Patent No. 3788750; Patent Literature 7: Japanese Patent No. 4144916). According to Patent Literatures 5 to 7, the physical foaming agent is introduced into a hermetically closed plasticizing cylinder, and the physical foaming agent is brought in contact with and dispersed in the plasticized and melted resin. The molten resin, in which the foaming agent is dispersed, is measured while maintaining the high pressure in the plasticizing cylinder to such an extent that the physical foaming agent is in the supercritical state, and then the molten resin is injected and charged into a mold. The supercritical fluid, which has been compatibly dissolved in the molten resin upon the injection and charging, is subjected to sudden pressure reduction and gasified. The molten resin is solidified, and thus foams (bubbles) are formed at the inside of the molded product.
The foam injection molding, which uses the supercritical fluid, is advantageous in that the process is clean, any residue of the foaming agent does not remain, and the mechanical strength of the molded product is hardly lowered because the foamed cell diameters become fine and minute, as compared with the foam injection molding which uses any chemical foaming agent. Further, the high pressure physical foaming agent functions as a plasticizer for the molten resin. Therefore, the following advantages are also provided. Namely, the viscosity of the resin is lowered upon the injection and charging, and the fluidity is improved; sink marks are suppressed, which would be otherwise caused by the shrinkage when the resin is solidified on account of the gas pressure upon the foaming; and the latent heat is deprived from the interior of the molten resin upon the foaming, and thus the cooling strain and the warpage are decreased.
Further, the supercritical fluid has a high density, and the supercritical fluid is measured (weighed) with ease. Therefore, the supercritical fluid is advantageous to stabilize the amount of introduction into the molten resin. For example, a method of Patent Literature 6 has been disclosed as a technique for stably and quantitatively supplying the physical foaming agent as described above into the plasticizing cylinder. According to Patent Literature 6, the resin internal pressure is controlled by the pressure of a load cell connected to a screw, i.e., by the back pressure of the screw (screw back pressure), and thus the supply amount of the foaming agent is controlled. Patent Literature 7 discloses a system wherein the back pressure of a screw is raised, and the pressure of the forward end of the screw at which a physical foaming agent is dissolved is maintained at a pressure in the supercritical state so that the separation is suppressed between the resin and the physical foaming agent.
The conventional foam injection molding method (method for producing foamed molded product) involves a problem or task such that the amount of the foaming agent introduced into the molten resin varies or fluctuates in every shot. The following artifice has been made to quantitatively supply an amount of the foaming agent introduced into the molten resin in the conventional foam injection molding method. Namely, for example, the internal pressure in a portion, of the plasticizing cylinder, which is located in the vicinity of the introducing port for the physical foaming agent, is subjected to the feedback, and the amount of introduction of the physical foaming agent is determined so that the differential pressure, which is provided between the pressure subjected to the feedback and the pressure of the physical foaming agent to be introduced, is constant.
However, the pressure in the portion of the plasticizing cylinder subjected to the feedback is the pressure provided one shot before, wherein the feedback is not perform in real-time. Further, the pressure in the portion of the plasticizing cylinder subjected to the feedback is obtained by detecting the screw back pressure. The amount of dissolution of the foaming agent is not completely uniform in the plasticizing cylinder. Therefore, the pressure in the plasticizing cylinder is sometimes distributed while providing the pressures different from the screw back pressure to be detected. Therefore, the amount of the foaming agent actually introduced into the molten resin is varied or fluctuated for every shot (between the shots), and it has been impossible to control the amount of the foaming agent.
Further, the conventional foam injection molding method involves another problem or task such that a measuring operation for measuring the molten resin becomes unstable including, for example, fluctuation in a time during which the molten resin is plasticized and measured (plasticizing and measuring time) and fluctuation in an amount of resin plasticized and measured (amount of resin to be charged into the mold). In the conventional foam injection molding method, it is necessary that the screw back pressure is set to be lower than an introducing pressure of the physical foaming agent into the plasticizing cylinder in order to introduce the physical foaming agent into the inside the plasticizing cylinder, and further that the introducing pressure of the physical foaming agent is set to be high (to be set as a high pressure) so that a large number of foaming nuclei are formed in the molten resin. In a case that a high pressure physical foaming agent is introduced inside the plasticizing cylinder, the resin pressure at a forward end (tip) portion of the plasticizing cylinder becomes higher than the screw back pressure in some cases. In such cases, the screw is suddenly pushed back toward a back (rear) portion of the plasticizing cylinder due to the resin pressure at the forward end portion of the plasticizing cylinder, which in turn makes the measurement operation (weight operation) for measuring (weighing) the molten resin by the screw be unstable. Further, in the conventional foam injection molding method, the differential pressure which is provided between the introducing pressure of the physical foaming agent and the screw back pressure is set to be small for the purpose of preventing the physical foaming agent from being introduced in any excessive amount. In this case, the introducing pressure of the physical foaming agent is set to be high; the screw back pressure is set also to be high. In a case that the screw back pressure is high, the screw can hardly retreat or move backward during a time wherein the plasticization and measuring is performed (during the plasticizing and measuring time), which in turn also makes the measuring operation for measuring the molten resin by the screw to be unstable. As discussed above, in the conventional foam injection molding method, the introducing pressure of a high pressure physical foaming agent affects the entire pressure in the plasticizing cylinder, and the screw back pressure is controlled based on the introducing pressure, consequently making the measuring operation for measuring the molten resin be unstable.
An object of the present teaching is to constantly stabilize the amount of a physical foaming agent to be introduced for each shot in a method of producing a foamed molded product including bringing in contact and kneading a molten resin with the physical foaming agent inside a plasticizing cylinder. Another object of the present teaching is to stabilize the measuring operation for measuring the molten resin by a screw in the plasticizing cylinder.